WDM optical communications systems that can carry information at rates up to terabits per second are becoming the next wave in optical communications development. In current WDM systems, information is optically coded within each of the WDM channels and the network is linked using a point-to-point architecture. Signal routing and switching are performed electronically (i.e., optical information is translated back to electronic format and then processed at each network node). As data rates increase, these opto-electronic and electro-optic conversions are becoming the bottleneck for the network. To improve the efficiency and reduce the cost of networks, routing and switching performed in the optical domain are preferred.
A wide variety of electromagnetic field-controlled optical switches are commercially available. They are based on mechanical, electro-optic, thermo-optic, acousto-optic, magnetooptic, and semiconductor technologies. Each switching technology has its own advantages, but also has drawbacks as well. For example, mechanical switches are the most widely used routing components and provide very low insertion loss and crosstalk characteristics, but their switching time is limited to the millisecond range. They also have a limited lifetime because motor-driven parts are used. LiNbO3 integrated optic switches, on the other hand, offer nanosecond switching times. However, LiNbO3 switches suffer from the disadvantages of relative large insertion loss (5 dB), high crosstalk (20 dB) and polarization dependency.
Accordingly, efforts continue to develop field-controlled optical switches with lower channel crosstalk, reduced polarization dependent loss, and at least moderate reconfiguration speed. It is recognized that these efforts, when successful, can provide an essential component to fiber communication systems.
A common optical element is a polarization beamsplitter PBS, as shown in FIG. 1. The PBS 10 is comprised of a reflection surface 11 located between two prisms 12, 13. PBS 10 is typically formed by coating the hypotenuse surface of one of the two prisms 12, 13. The hypotenuse surface of the other prism 13, 12 is then attached, via an optical adhesive, to the coated surface. The PBS operates by splitting an incoming beam 14 into its polarization components, specifically the horizontal component 15 (which is also referred to as the p component and sometimes represented as ".vertline.") and vertical component 16 (which is also referred to as the s component and sometimes represented as ".cndot."). With an ideal PBS, all of the p light 15 is transmitted (or passes through) and all of the s light 16 is reflected. However, typically PBSs leak light in both directions. In other words some p light is reflected, i.e. p.sub.L 17, and some s light is transmitted, i.e. s.sub.L 18. The ratios of the reflected s light 16 to the leaked, reflected p light RS:Rp.sub.L, and the transmitted p light 15 to the leaked, transmitted s light 18, Tp:Ts.sub.L, are known as extinction ratios. Typical PBSs have extinction ratios of Rs:Rp.sub.L of about 20 dB and Tp:Ts.sub.L of about 40 dB. The higher the extinction ratio, the better the PBS. Consequently the transmitted extinction ratio Tp:Ts.sub.L is better than the reflected extinction ratio RS:Rp.sub.L, which is typical for PBSs. However, note that as long as the leaking signal has a polarization that is different from the actual signal, e.g. s light 16 has a different polarization from the leaking p light, p.sub.L 17, then the leaking light or noise can be filtered off of the signal with a polarization filter or polarizer.
The leakage problem or low extinction ratio problem becomes particularly relevant in optical switching systems, wherein a single PBS has two inputs, for example as shown in FIG. 2. Note that in FIG. 2 only the p light is shown for the sake of simplicity, but both inputs may also comprise s light. As shown in FIG. 2, a first input p.sub.1 signal 21 is incident on to one surface of PBS 20, and has a transmitted portion p.sub.1 23, which is the intended signal, and a noise portion p.sub.1L 25, which represents a reflected portion of the p.sub.1 input signal 21. The second input p.sub.2 signal 22 is incident on a second surface of the PBS 20, and has a transmitted portion p.sub.2 24, which is the intended signal, and a noise portion p.sub.2L 26, which represents a reflected portion of the p.sub.2 input signal 22. Note that the cross point 27 (or intersection) of two input signals is located at the beamsplitting surface 28. In this arrangement, the p.sub.2 signal 24 and p.sub.1L noise 25 have the same polarization and are co-linear. Similarly, the p.sub.1 signal 23 and the p.sub.2L noise 26 have the same polarization and are co-linear. Consequently, the noise signals cannot be removed from the overlapping signals. Therefore, at each cross point of two input signals, crosstalk noise is injected into the signals. While the noise injected by a single cross point may not be too high for system operations, typical WDM systems may have dozens or even hundreds of cross points, thus noise will quickly accumulate and swamp the input signals. As a result, prior art PBSs are not effectively used in WDM systems.
Therefore, there is a need in the art for a high extinction ratio beamsplitter, such that crosstalk noise is not injected into the input signals.